One Transistor Code Lock

This is of course the simplest electronic code lock circuit one can make. The circuit uses one transistor, a relay and few passive components. The simplicity does not have any influence on the performance and this circuit works really fine. The circuit is nothing but a simple transistor switch with a relay at its collector as load. Five switches (S0 to S4) arranged in series with the current limiting resistor R2 is connected across the base of the transistor and positive supply rail.

Another five switches (S5 to S9) arranged in parallel is connected across the base of the transistor and ground. The transistor Q1 will be ON and relay will be activated only if all the switches S0 to S4 are ON and S5 to S9 are OFF. Arrange these switches in a shuffled manner on the panel and that it. The relay will be ON only if the switches S0 to S9 are either OFF or ON in the correct combination. The device to be controlled using the lock circuit can be connected through the relay terminals. Transformer T1, bridge D1, capacitor C1 forms the power supply section of the circuit. Diode D2 is a freewheeling diode. Resistor R1 ensures that the transistor Q1 is OFF when there is no connection between its base and positive supply rail.

Circuit diagram:

Notes:

• This circuit can be assembled on a Vero board.
• Switch S1 is the lock’s power switch.
• The no of switches can be increased to make it hard to guess the combination.
• Transistor 2N2222 is not very critical here. Any low or medium power NPN transistor will do the job.

Low-Cost Low-Intensity Alarm

Ever needed an alarm to monitor something but you didn't want to annoy the neighbours or scare the hell out of the family in the middle of the night? Using a feature found in most smoke alarms these days, this little circuit may do the trick for you. When the 9V battery level drops to about 7.4V in most smoke alarms, they emit a brief chirp at around 40 second intervals.

In most cases, this is enough to attract attention, without the likelihood of waking the whole household. In effect, this circuit simply powers the smoke alarm at 6.8V, as derived from a zener diode (ZD1), so that it sounds its low battery warning. Apart from the zener diode, the smoke alarm draws only a few microamps, with a brief current spike during the "chirp". Smoke alarms are now very cheap to buy so this is a good alternative to a piezo siren.

Circuit diagram:

More information on a common Smoke Alarm chip made by Motorola can be obtained at: http://e-www.motorola.com/files/sensors/doc/data_sheet/MC14467-1.pdf

Laser Alarm

This circuit is a laser alarm system like the one we see in various movies. It uses a laser pointer beam to secure your valuables and property. Essentially, when the beam gets interrupted by a person, animal or object, the resistance of a photodiode will increase and an alarm will be activated. The laser and the receiver can be fitted in same box, sharing a common power supply. As the receiver draws less than 10 mA on average, you’ll soon find that the laser is the most current hungry device! Mirrors are used to direct the beam in whatever setup you require. Examples of a passage and an area protected by the alarm are shown in the diagram.

In the circuit diagram we find a TL072 op-amp (IC1.A) configured as voltage comparator between the voltage reference provided by the adjustable voltage divider P1/R4 and the light-dependent voltage provided by the voltage divider consisting of photodiode D1 and fixed resistor R3. When the laser beam is interrupted, the voltage on comparator pin 2 drops below that at pin 3, causing the output to swing to (almost) the positive supply voltage and indicating an alarm condition. This signal can drive a siren, a computer or a light that hopefully will deter the intruder.

Circuit diagram:

Alternatively it can be used to ‘silently’ trigger a more sophisticated alarm. Resistor R2 provides some hysteresis to prevent oscillation when the two comparator input voltages are almost equal. Capacitor C1 makes the circuit immune to short, accidental interruptions of the beam, e.g., by flying insects. If you want your circuit to have faster responses you can reduce its value to 1 µF. The operation of the circuit is illustrated by the waveform diagram, which also proves the hysteresis action that sets an upper and a lower threshold on the input voltage. You can also see the delay introduced by capacitor C1.

The circuit is simple and could be assembled on a piece of breadboard. After assembling the circuit and testing it, you should mount it in a black box that has just a small hole. You may decide to put the laser in the same box but only if you are sure there is no way the photodiode can ‘see’ the laser beam directly. The small hole should be filled with a black drinking straw so that only light from the direction of the laser beam can enter. With the appropriate setup of the box and the mirrors, the laser beam is so intense that even direct sunlight cannot affect the operation of the photodiode.

Author: Dimitris Kouzis-Loukas - Copyright: Elektor Electronics Magazine

Gated Alarm

Sometimes the need arises for a simple, gated, pulsed alarm. The circuit shown here employs just four components and a piezo sounder and is unlikely to be out-done for simplicity. While it does not offer the most powerful output, it is likely to be adequate for many applications. A dual CMOS timer IC type 7556 is used for the purpose, with each of its two halves being wired as a simple astable oscillator (a standard 556 IC will not work in this circuit, nor will two standard 555’s). Note that the CMOS7556 is supplied by many different manufacturers, each using their own type code prefix and suffix. The relevant Texas Instruments product, for instance, will be marked ‘TLC556CN’.

The circuit configuration used here is seldom seen, due probably to the inability of this oscillator to be more than lightly loaded without disturbing the timing. However, it is particularly useful for high impedance logic inputs, since it provides a simple means of obtaining a square wave with 1:1 mark-space ratio, which the ‘orthodox’ configuration does not so easily provide. IC1.A is a slow oscillator which is enabled when reset pin 4 is taken High, and inhibited when it is taken Low. Output pin 5 of IC1.A pulses audio oscillator IC1.B, which is similarly enabled when reset pin 10 is taken High, and inhibited when it is taken Low.

Circuit diagram:

In order to simplify oscillator IC1.B, piezo sounder X1 doubles as both timing capacitor and sounder. This is possible because a passive piezo sounder typically has a capacitance of a few tens of nanofarads, although this may vary greatly. As the capacitor-sounder charges and discharges, so a tone is emitted. The value of resistor R2 needs to be selected so as to find the resonant frequency of the piezo sounder, and with this its maximum volume. The circuit will operate off any supply voltage between 2 V and 18 V. A satisfactory output will be obtained at relatively high supply voltages, but do not exceed 18 V.
Author: Rev. Thomas Scarborough - Copyright: Elektor July-August

Model Railway Short-Circuit Beeper

Short circuits in the tracks, points or wiring are almost inevitable when building or operating a model railway. Although transformers for model systems must be protected against short circuits by built-in bimetallic switches, the response time of such switches is so long that is not possible to immediately localise a short that occurs while the trains are running, for example. Furthermore, bimetallic protection switches do not always work properly when the voltage applied to the track circuit is relatively low. The rapid-acting acoustic short-circuit detector described here eliminates these problems. However, it requires its own power source, which is implemented here in the form of a GoldCap storage capacitor with a capacity of 0.1 to 1 F. A commonly available reed switch (filled with an inert gas) is used for the current sensor, but in this case it is actuated by a solenoid instead of a permanent magnet.

An adequate coil is provided by several turns of 0.8–1 mm enamelled copper wire wound around a drill bit or yarn spool and then slipped over the glass tube of the reed switch. This technique generates only a negligible voltage drop. The actuation sensitivity of the switch (expressed in ampère-turns or A-t)) determines the number of turns required for the coil. For instance, if you select a type rated at 20–40 A-t and assume a maximum allowable operating current of 6 A, seven turns (40 ÷ 6 = 6.67) will be sufficient. As a rule, the optimum number of windings must be determined empirically, due to a lack of specification data. As you can see from the circuit diagram, the short-circuit detector is equally suitable for AC and DC railways. With Märklin transformers (HO and I), the track and lighting circuits can be sensed together, since both circuits are powered from a single secondary winding.

Coil L1 is located in the common ground lead (‘O’ terminal), so the piezoelectric buzzer will sound if a short circuit is present in either of the two circuits. The (positive) trigger voltage is taken from the lighting circuit (L) via D1 and series resistor R1. Even though the current flowing through winding L1 is an AC or pulsating DC current, which causes the contact reeds to vibrate in synchronisation with the mains frequency, the buzzer will be activated because a brief positive pulse is all that is required to trigger thyristor Th1. The thyristor takes its anode voltage from the GoldCap storage capacitor (C2), which is charged via C2 and R2.

The alarm can be manually switched off using switch S1, since although the thyristor will return to the blocking state after C2 has been discharged if a short circuit is present the lighting circuit, this will not happen if there is a short circuit in the track circuit. C1 eliminates any noise pulses that may be generated. As a continuous tone does not attract as much attention as an intermittent beep, an intermittent piezoelectric generator is preferable. As almost no current flows during the intervals between beeps and the hold current through the thyristor must be kept above 3 mA, a resistor with a value of 1.5–1.8 kΩ is connected in parallel with the buzzer. This may also be necessary with certain types of continuous-tone buzzers if the operating current is less than 3 mA. The Zener diode must limit the operating voltage to 5.1 V, since the rated voltage of the GoldCap capacitor is 5.5 V.

Author: R. Edlinger - Copyright: Elektor Electronics